Over the last few decades, the electronics industry has undergone a revolution by the use of semiconductor technology to fabricate small, highly integrated electronic devices. The most common semiconductor technology presently used is silicon-based. A large variety of semiconductor devices have been manufactured having various applications in numerous disciplines. One such silicon-based semiconductor device is a metal-oxide-semiconductor (MOS) transistor.
The principal elements of a typical MOS semiconductor device are illustrated in FIG. 1. The device generally includes a semiconductor substrate 101 on which a gate electrode 103 is disposed. The gate electrode 103 is typically a heavily doped conductor having uniform conductivity. An input signal is typically applied to the gate electrode 103 via a gate terminal (not shown). Heavily doped source/drain regions 105 are formed in the semiconductor substrate 101 and are connected to source/drain terminals (not shown). As illustrated in FIG. 1, the typical MOS transistor is symmetrical, which means that the source and drain are interchangeable. Whether a region acts as a source or drain depends on the respective applied voltages and the type of device being made (e.g., PMOS, NMOS, etc.). Thus, as used herein, the term source/drain region refers generally to an active region used for the formation of a source or drain. A channel region 107 is formed in the semiconductor substrate 101 beneath the gate electrode 103 and separates the source/drain regions 105. The channel is typically lightly doped with a dopant type opposite to that of the source/drain regions 105. The gate electrode 103 is generally separated from the semiconductor substrate 101 by an insulating layer 109, typically an oxide layer such as SiO.sub.2. The insulating layer 109 is provided to prevent current from flowing between the gate electrode 103 and the source/drain regions 105 or channel region 107.
In operation, an output voltage is typically developed between the source and drain terminals. When an input voltage is applied to the gate electrode 103, a transverse electric field is set up in the channel region 107. By varying the transverse electric field, it is possible to modulate the conductance of the channel region 107 between the source region and the drain region. In this manner an electric field controls the current flow through the channel region 107. This type of device is commonly referred to as a MOS field-effect-transistor (MOSFET).
Semiconductor devices, like the one described above, are used in large numbers to construct most modern electronic devices. As a larger number of such devices are integrated into a single silicon wafer, improved performance and capabilities of electronic devices can be achieved. In order to increase the number of semiconductor devices which may be formed on a given surface area of a substrate, the semiconductor devices must be scaled down (i.e., made smaller). This is accomplished by reducing the lateral as well as vertical dimensions of the device structure.
As device geometries are scaled down, fabrication of the device becomes more complicated. For example, as transistors are scaled, the transverse electric field generated in a MOS device typically increases. If the transverse electric field becomes sufficiently strong, it can give rise to hot carrier effects which can significantly degrade device performance. The problems associated with hot carrier effects are particular pronounced in short channel devices (having, for example, submicron channel lengths) and serve to limit scaling down of semiconductor devices.
One important hot carrier effect is hot-carrier injection of electrons into the gate oxide and/or gate electrode. Hot carrier injection generally causes a deleterious gate current (when the electrons pass into the gate electrode) and an undesirable increase in the threshold voltage of the device (when the electrons are trapped in the gate oxide). Another important hot carrier effect is forward injection of electrons from the source to the drain. Forward injection of electrons generally causes a deleterious source-to-drain current (often referred to as a leakage current).